1. Field of the Disclosure
The present disclosure relates to the field of liquid crystal display, and more particularly, to a pixel driving circuit and an organic light-emitting diode (OLED) display device.
2. Description of Related Art
An OLED display device of related art is recognized by the industry to have an extremely developmental potential because the OLED display device has advantages of compactness, simple structure, self-illumination, high brightness, wide viewing angle, short response time, etc.
The OLED device is an electric current driven device. The electric current flows through the OLED to make the OLED give out light. The brightness of the OLED is determined by the electric current. Most integrated circuits (ICs) of related art merely transmits voltage signals so the pixel driving circuit of the OLED needs to transforming a voltage signal into an electric current signal. The pixel driving circuit of related art is a 2T1C pixel with the structure of two TFTs and one capacitor. With the structure, the pixel driving circuit can transform the voltage into the electric current. However, the pixel driving circuit of related art does not have the function of compensation.
FIG. 1 is a circuit diagram of a 2T1C pixel driving circuit of related art. The 2T1C pixel driving circuit includes a first thin-film transistor (TFT) T10, a second TFT T20, and a capacitor Cs. The first TFT T10 is a driving TFT. The second TFT T20 is a switch TFT. The capacitor Cs is a storage capacitor. Specifically, agate of the second TFT T20 is electrically connected to a scanning signal Vsel. A source of the second TFT T20 is electrically connected to a data signal Vdata. A drain of the second TFT T20 is electrically connected to agate of the first TFT T10. A drain of the first TFT T10 is electrically connected to a power signal Vdd. A source of the first TFT T10 is electrically connected to an anode node level of the OLED D. A cathode of the OLED D is electrically connected to a ground terminal. One terminal of the capacitor Cs is electrically connected to a drain of the second TFT T20. The other terminal of the capacitor Cs is electrically connected to the source of the first TFT T10.
Please refer to FIG. 2 as well. FIG. 2 illustrates a timing diagram of a 2T1C pixel driving circuit as illustrated in FIG. 1. The working process of the 2T1C pixel driving circuit is divided into a first working stage S10 and a second working stage S20. At the first working stage S10, the data signal Vdata is a display data signal VDATA, and the scanning signal Vsel is at high voltage level. At the first working stage S20, the data signal Vdata is at low voltage level, and the scanning signal Vsel is at low voltage level as well. At the first working stage S10 and the second working stage S20, the power signal Vdd is constant high voltage.
In the present embodiment, the OLED D gives out light at the second working stage S20. Meanwhile, a gate-source voltage Vgs imposed on the first TFT T10, which drives the OLED D to light, satisfies the following equation:Vgs=VDATA−VOLED;
where VDATA indicates the data signal at high voltage level, and VOLED indicates voltage level of an anode node of the OLED D.
As the equation tells, the gate-source voltage Vgs for driving illumination of the OLED D is irrelevant to the threshold voltage imposed on the first TFT T10. Therefore, the 2T1C pixel driving circuit fails to compensate the threshold voltage imposed on the driving TFT, i.e., the first TFT T10.